A Capacitance-to-Time Converter-Based Electronic Interface for Differential Capacitive Sensors

Abstract

In this paper we present an oscillating conditioning circuit, operating a capacitance-to-time conversion, which is suitable for the readout of differential capacitive sensors. The simple architecture, based on a multiple-feedbacks structure that avoids ground noise disturbs and system calibrations, employs only three Operational Amplifiers (OAs) and a mixer implementing a square wave oscillator that provides an AC sensor excitation voltage. It performs a Period Modulation (PM) and a Pulse Width Modulation (PWM) of the output signal proportionally to the sensor differential capacitance values. The sensor variation range and the detection sensitivity can be easily set through the additional resistors. Preliminary PSpice simulation results have shown a good agreement with theoretical calculations as well as a linear response with a high detection sensitivity of differential capacitive sensors having a baseline in the range [2.2 ÷ 180 pF]. Moreover, different experimental measurements have been also performed by implementing the circuit on a laboratory breadboard using commercial discrete components so validating the idea and providing the circuit performances with different kind of differential capacitive sensors achieving detection resolutions of about 0.1 fF in an overall differential capacitive variation range that is equal to ±15.8 pF. The achieved results demonstrate that the proposed interface solution is suitable for on-chip integration with different kinds of differential capacitive sensing devices, such as Micro-Electro-Mechanical-System (MEMS), force/position, and humidity sensors in biomedical and robotics applications.

A Highly Robust Interface Circuit for Resistive Sensors

“…Both simulation and experimental data, as well as the linear Equation (5), using the weighted least square regression, are shown in Figure 10. The circuit was tested with Vdd equal to 5 V. A linear correlation coefficient of 0.9953 was obtained with respect to Equation (5). Vertical error bars show ±2σ.…”

“…This results in large and complex configurations and linearization techniques must be applied due to the intrinsic limitation in the dynamic range [2][3][4]. In contrast, quasi-digital converters, i.e., resistance-to-frequency [5,6], -period [7,8] or -duty-cycle [9] converters, are preferred if the resistance variations are very large. This converters not only provide a wider dynamic range but also simplify interfacing to digital systems, as no analog-to-digital converters (ADCs) are required [10,11].…”

Microdroplet-Based Organic Vapour Sensor on a Disposable GO-Chitosan Flexible Substrate

“…For that reason, different quasi-digital converters have been reported in literature including resistance-toduty cycle, resistance-to-frequency, and resistance-to period, as they are simpler topologies, offering a wide dynamic range and a simplified interface to digital systems (e.g. microprocessors) [40]- [44].…”

Signal Processing and Analysis of Electrical Circuit

“…The proposed interface is suitable for on-chip integration with sensors of force, humidity, position etc. [9]. The self-calibrating dynamic comparator was developed.…”